Email send goes to Outbox.

I get Error 550 on email sends. Just started happening. Was working fine and I of course changed nothing. I contacted ISP and confirmed that the setting were correct. POP3 SMTP.charter.net and Out Port is 25.
I can send email if I use their Email service but is cumbersome to use.
When I try again using LM it goes to Outbox?

From PackGy. I noticed that my Outbox was building with mail. When I'd shut down I'd get the message, "do you want to send mail in the Outbox?" It kept pointing to the same email as the culprit. I 'assumed' it would pass by a problem email and go on to the next one. It appears it does them one after another in sequence. If first one has a problem it stops. No attempt made on any other one. I know this because I finally just deleted the one it kept mentioning at... Then all the others were sent! Problem is gone! Hope this helps someone else.

I've gotten this a few times and it usually is caused by an invalid email address. Most often it was when I entered a space somewhere in an email address like right after the @ sign or before or after the dot. This sometimes happens with my very sensitive touchpad.